1986.5 Automatic Shifting High (and sometimes low?)
 

My wife's 1986.5 Supra with an auto trans has a weird issue. It decided out of the blue to start shifting up at about 4500 RPM during normal acceleration. Putting the ETC into sport mode changes that to about 5000 RPM. It kinda feels like it's shifting a bit hard as well. I ran through the tests in the repair manual and it passed all of them (it was a few years ago, so I don't entirely remember them. I used a jumper wire at one point, definitely. The car has spent a lot of time rotting away in a shop since then). Unplugging the ECT (down by the passenger's foot) allowed manual shifting, which works fine.

Today, something weird happened, though. I replaced the battery last night (not sure if it's related, but I thought I'd mention it) and on my way home from work today, I thought I'd give it a shot and plug the ECT computer in, and it shifted at about 2500 RPM. I had thought it was "fixed", or at least somewhat. Flooring it didn't cause the Supra to downshift at all and it really didn't seem to affect shifting whatsoever. I stopped the car when I got home, did some work on the steering wheel (damn indicator needed cleaning like none other), and it was back to 4500 RPM shifts after.

What the heck is going on? Would it be the speedometer cable? I've searched and haven't really found anything. It does this between ALL gears, not just a few. I'd think it'd be something electrical rather than mechanical, but my experience with automatics isn't so great.

Any cheap fixes? Ripping out the auto and replacing it with a manual isn't an option, sadly. Thanks in advance

Does the speedometer still work? The electronic speed signal is generated by the speedometer, without it the ecu and ect do not know what speed the car is moving.

The shifting at 4500 RPM's reminds me of my first Supra. After it overheated (which ruined the engine and caused me to replace it with a JDM engine), the car would not shift below that same engine speed, regardless of amount of throttle. My car also would not downshift properly. The cause turned out to be 2 seperate problems: I had a melted throttle position sensor (from overheating), and an improperly adjusted transmission kick down cable. I would recommend checking both the tps and kickdown cable for proper function and calibration (see the tsrm, you can find a link in the "Helpful Supra Websites" thread in the MKIII FAQ forum).

Yes, the speedometer still works. It was a hunch in the wrong direction. I'll take a look at those later today. Also, I was looking at this trying to find out which tests I had done previously, and I think I did all of the tests up to (and including) page AT-20. I'd been working on it all day and I finally had a "fix" with manual shifting and left it at that.

So I tried the steps to test the TPS here and it seems the TPS is shot. Ordered a new one; I'll update once I've replaced it.

Replaced the TPS and she runs like new again. Thanks!